The transport of large parts of a sea platform, such as a support structure or a top side as a whole, is required during both the removal and installation thereof. There is a desire to transport the topside or supporting structure as a complete part to limit offshore installation work.
Typically, existing sea platforms have a certain economical or technical life span after which they need to be removed. The removal generally comprises a lift operation and a transport operation.
In this document, the term “lifting operation” comprises all operations before the actual lift off of the part of the sea platform to be removed, e.g. engagement of the clamping system and the load transfer until the lift off, and the lift off procedure itself. The term “transport operation” comprises the operations which start after the lift off of the upper part has finished, i.e. after the upper part of the sea platform is lifted from the lower part and supported by the lifting vessel.
The lift operation is a complex and expensive operation for several reasons. One reason is the shear size and weight of sea platforms. Only very large lifting vessels are capable of lifting such a platform in a single lift or in a limited number of lifts.
Another reason is the fact that a lifting vessel moves under influence of waves, wind and current. The sea platform generally is stationary or substantially stationary, because it rests on the sea bottom or is limited in its motions in another way. The lifting vessel therefore moves with respect to the sea platform which makes it difficult to connect to the sea platform and to carry out the lifting operation. There generally is a risk of collision between the lifting vessel and the sea platform.
It was recognized in the present invention that it is beneficial to have the capability of switching between a mode having a low vertical stiffness to a mode having a high 10 vertical stiffness because during the lifting operation there needs to be more relative freedom of movement than in the subsequent transport operation. It was recognized that this switching between modes should be very reliable and as simple as possible.
A number of heavy lift systems have been developed so far. One known concept is to use large cranes on a semi-submersible vessel. This solution is known from existing heavy lift vessels such as Hermod, Balder and Thialf. These vessels can lift an upper part of a sea platform from a lower part and subsequently place the upper part on a barge. It was found that this is a rather complex operation which requires calm weather. The lift capacity is also limited.
Another concept is disclosed in US2013/0045056. A lifting vessel has two arms and a number of lifting devices on these arms. It was recognized in the present invention that the lifting device of US2013/0045056 allows only a limited freedom of movement of the lifting vessel relative to the sea platform. This narrows a window of opportunity in which the weather is sufficiently calm for the operation to be carried out in many locations.
Further, known lifting systems rely on an accurately pre-calculated centre of gravity (COG) of the sea platform. If during the lift operation the position of the COG is found to be different than expected or calculated on the basis of the available data, a compensation system is required to cope with the difference in load distribution between the connection points.
Another lifting vessel system is being developed under the name Pioneering Spirit by the company Allseas, a description of which can be found on www.allseas.com. It was recognized in the present invention that this system requires large actuators to compensate for the relative movements between the sea platform and the lifting vessel. The large actuators are complex and require much maintenance. Moreover, during the actual lifting operation the actuators are actively controlled which requires much energy to keep the movements of the sea platform within the prescribed limits.
A variant of the system of Pioneering Spirit is disclosed in GB2306407 in the name of Allseas Group. This concept is not capable of switching between a mode having a low vertical stiffness to a mode having a high vertical stiffness. Furthermore, the connectors of GB2306407 which connect to the topside are simple abutment faces. This is a rather primitive concept. It was recognized in the present invention that connectors which can slide relative to leg parts of the structure are more advanced.
Another lifting concept is disclosed in WO0172582 in the name of Excalibur Engineering B.V. This concept is also not capable of switching between a mode having a low vertical stiffness to a mode having a high vertical stiffness.
A further concept is published on 29 Jan. 2015 in WO2015/012695 in the name of Heerema Marine Contractors. This concept is rather complex and requires a cumbersome transition from a mode having a low vertical stiffness to a mode having a high vertical stiffness. Furthermore, this system results in high tensions or excessive relative movements during transport.
A further concept is disclosed in GB2363814 in the name of Marine Shuttle Operations AS. Like WO0172582, this concept does not provide the capability of switching between a mode having a low vertical stiffness to a mode having a high vertical stiffness. Furthermore, this concept engages the topside directly, instead of engaging leg parts underneath the topside. This is very disadvantageous.